1. Field of the Invention
The present invention relates to polycarbonate compositions which have been provided with permanent antistatic properties and which, compared with the prior art, have improved heat distortion resistance, low-temperature strength, processing stability, stress cracking resistance and, in particular, hydrolytic stability. The present invention relates additionally to processes for the preparation of these compositions and to their use.
2. Description of Related Art
The accumulation of dust with the formation of dust figures is a widespread problem in moulded plastics articles. See in this connection, for example, Saechtling, Kunststoff-Taschenbuch, 26th edition, Hanser Verlag, 1995, Munich, p. 140 f. Dust deposits on high-gloss moulded articles are particularly troublesome and functionally limiting. Such dust accumulations are particularly disruptive when the moulded articles are also to be lacquered, so that the dust can adversely affect the optical result.
A known method of reducing the accumulation of dust on plastics articles is the use of antistatic agents. Antistatic agents for thermoplastics which limit the accumulation of dust are described in the literature (see e.g. Gächter, Müller, Plastic Additives, Hanser Verlag, Munich, 1996, p. 749 ff). These antistatic agents reduce the electric surface resistance of the plastics moulding compositions, as a result of which surface charges which form by friction during production and use are better dissipated. Dust particles are accordingly subjected to less electrostatic attraction, and consequently there is less accumulation of dust.
In the case of antistatic agents, a distinction is generally made between internal and external antistatic agents. An external antistatic agent is applied to the moulded plastics article after processing, while an internal antistatic agent is added to the plastics moulding compositions as an additive. The use of internal antistatic agents is usually desirable for economic reasons, because further cost-intensive working steps for the application of the antistatic agent are not required after processing.
A frequently described class of internal antistatic agents in thermoplastics are the salts of long-chained sulfonic acids. In polycarbonate and polyester compositions, however, these compounds, like the long-chained quaternary phosphonium or ammonium salts which are likewise used as antistatic agents, lead to molecular weight degradation of the polymer and hence to an impairment of the application-related properties (heat distortion resistance, stress cracking resistance, ultimate elongation, toughness, etc.) under the thermal load to which they are subjected during their preparation and processing. Such low molecular weight anionic or cationic surface-active antistatic agents, like other low molecular weight non-ionic antistatic agents such as, for example, fatty acid esters, ethoxylated alkylamines, ethoxylated alcohols or diethanolamides, act via their migration to the surface of the plastic, where they attract water from the air, as a result of which the surface conductivity of the plastic is ultimately increased. In this regard, a further disadvantage of these compounds is that these antistatic agents do not develop their action sufficiently under conditions with low atmospheric humidity, the antistatic action does not start immediately after the production of the components and, above all, is not of a permanent nature. In particular, these surface-active antistatic agents can be partly or completely removed from the surface of the component again by cleaning of the surfaces, for example, which ultimately lowers the antistatic action permanently.
Polyether amides, polyester amides or polyether ester amides are likewise known as antistatic agents, these compounds being capable of providing polymer compositions with a permanent antistatic action. Their action is based on the formation of a three-dimensional network of the antistatic agent in the polymer which is to be provided with antistatic properties and which is immiscible with the antistatic agent. Owing to the comparatively high electrical conductivity of the polyether amides, polyester amides and polyether ester amides, there is an immediate and permanent reduction in the electric surface and volume resistance of the polymer which is to be provided with antistatic properties, which reduction occurs even in the case of very low atmospheric humidity and is far less dependent thereon than is the case with the surface-active compounds mentioned above.
EP 613919, U.S. Pat. No. 6,784,257, U.S. Pat. No. 6,706,851 and U.S. Pat. No. 6,913,804 disclose polymer compositions provided with antistatic properties by means of polyether amides, polyester amides or polyether ester amides. The addition of acids is not mentioned in these applications.
A disadvantage of the use of polyether amides, polyester amides and polyether ester amides as antistatic agents in polycarbonate or polyester compositions is that their use under the thermal load to which such compositions are subjected during their preparation and processing leads to molecular weight degradation of the polycarbonate or polyester and accordingly to an impairment of the application-related properties (heat distortion resistance, stress cracking resistance, toughness, processing stability and hydrolytic stability, etc.) of the compositions.
From JP 09137053 A there are known polycarbonate compositions which are provided with antistatic properties by means of polyether amides, polyester amides and/or polyether ester amides and the antistatic properties of which are enhanced by addition of from 0.01 to 10 wt. % of an inorganic or organic electrolyte. There are mentioned as electrolytes inorganic and organic salts and dodecylbenzenesulfonic acid, p-toluenesulfonic acid, dodecyl phenyl ether disulfonic acid, naphthalenesulfonic acid and condensation products thereof with formalin, arylsulfonic acids, alkylsulfonic acids, stearic acid, laurylic acid and polyacrylic acids as well as organic phosphoric acids. In principle, however, the teaching of JP 09137053 A leads away from the use of acids because the results achieved therewith are poorer than the results achieved with the corresponding salts.